FIG. 1 shows a simplified cross-section of a typical transmissive active-matrix liquid crystal display (AMLCD). The backlight 10 serves as a light source for illumination of the display. The transmission of light through the display, from the backlight 10 to the viewer 20, is controlled by the use of electronic circuits made from thin film transistors (TFTs). These TFTs are fabricated on a glass substrate (known as the TFT glass 30) and are operated in conjunction with a ‘counter-electrode’ 40 so as to vary the local electric field appearing across the liquid crystal (LC) layer 50. The local electric field dictates the optical properties of the LC material, and thus permits selective transmission of light from the backlight 10 through to the viewer 20.
If an electric field of non-zero mean is maintained over the LC material, it will suffer degradation. As the transmission of light through the LC material is dictated by the magnitude of the applied field, rather than its polarity, it is acceptable to periodically invert the polarity of the field so as to achieve a zero mean. Typically, this inversion is accomplished by varying both the voltages present on the TFT glass 30 and that present on the common counter-electrode 40. To this end, the counter-electrode 40 is typically driven with a square wave of several volts. The counter-electrode 40 signal may therefore act as a source of electrical interference to any other circuits integrated within the display.
The signal supplied to the counter-electrode 40, often termed ‘VCOM’, is generated by electronics incorporated into the display module. An integrated circuit (IC) in the host product (for example the mobile phone or portable computer which contains the display) generates further necessary display signals, such as HSYNC, CK and RGB. Examples of these signals can be found in FIG. 2.
Many products which contain displays (e.g., mobile phones and portable computers) benefit from an ability to control the intensity of the backlight according to conditions of ambient illumination. For example, under low ambient lighting conditions it is desirable to reduce the brightness of the display by reducing the intensity of the backlight. This serves to minimize consumption of power by the backlight, and prevents the user from suffering ‘glare’.
In order to vary the intensity of the backlight in accordance with ambient lighting conditions, it is necessary to have some means for sensing the level of ambient light. An ambient light sensor (ALS) is used for this purpose, and there are advantages to integrating the ALS onto the TFT glass 30 (termed ‘monolithic integration’). These advantages include reduction of the product's size, weight and manufacturing costs.
A typical ambient light sensor system, as shown in FIG. 3, contains the following elements:                (a) A photodetection element (or elements) capable of converting incoming light to electrical current. An example of such a photodetection element is a photodiode 60.        (b) Voltage bias generating and current measurement circuitry to control the photodetection element(s) and sense the photo-generated current 70. This circuitry will typically take the form of an integrator.        (c) Output circuitry 80 to supply an output signal (analog or digital) representing the measured ambient light level.        (d) A means of adjusting the display operation 90 based on the measured ambient light level, for example by controlling the intensity of the backlight 10.The operation of such components is described in WO2008/044749A1.        
In the case of an AMLCD having a monolithically integrated ambient light sensor, the photodetection device used must be compatible with the TFT process employed in the manufacture of the TFT glass 30. A well-known photodetection device compatible with the standard TFT process is the lateral thin-film poly-silicon P-I-N diode: a two terminal device having a cathode 100 and an anode 110, whose circuit representation is shown in FIG. 4.
The current flowing in the P-I-N diode is a function of three quantities: temperature, the amount of illumination incident upon the diode, and the potential difference which appears across its terminals (the ‘bias voltage’). The asymmetric relationship between diode current (Iac) and applied bias voltage (Vac) is shown in FIG. 5 (for the case of zero illumination). The current which flows on account of a non-zero bias voltage is detrimental to the operation of an ambient light sensor, as it cannot be distinguished from that which flows due to incident illumination. This component of the diode current is termed the ‘dark current’, and may be significantly temperature dependent.
As the dark current may scale exponentially with the photodiode bias voltage, it is desirable to minimize this applied bias. Ideally, the diode's terminals would be maintained at identical potentials. However, such precision voltage control can be difficult to achieve in practice. It is therefore wise to series connect many photodiodes, so as to divide any applied potential across numerous devices. FIG. 6 shows a series connected photodiode stack, as described in WO2008/044749A1, finding use in an ALS circuit. The stack of n series connected photodiodes 120 is connected to bias generating and current measurement circuitry 70. The bias generating and current measurement circuitry 70 measures diode current whilst maintaining a constant (and ideally zero) bias voltage across the diode stack 120. Any residual bias voltage, VB, appearing across the diode stack 120 is divided between the individual devices, such that each photodiode experiences a lesser bias of VB/n. Dark current from the photodiode is therefore reduced.
However, series connected diodes have a disadvantage when employed within an electrically noisy environment. As shown in FIG. 7, electrical interference from the counter-electrode 130 will couple to points within the diode stack 120, via parasitic coupling capacitances 140. Normally, the effect of electrical interference in sensor circuits may be reduced by techniques such as low pass filtering or averaging of the sensor output. Unfortunately, the asymmetric conduction characteristic of the diode, shown in FIG. 5, may cause a net forward current to flow in the presence of electrical interference. This net forward current cannot be cancelled out by time-averaging or integration, and cannot be separated from the light dependent diode current.
This undesirable effect is explained as follows, with reference to FIG. 8. Upon a rising edge of the counter-electrode voltage, positive voltage steps will be seen at each point throughout the stack of series connected diodes 120. However, the anode and cathode ends of the stack 120, connected to ground and to the bias circuitry respectively, will be maintained at constant voltage. The diode 150 at the cathode-end of the series connected stack 120 will therefore become temporarily forward biased, and will pass a light-independent current in the forward direction. At the same time, the diode 160 at the anode-end of the series connected stack 120 will become temporarily reverse biased, yet will pass little current in the reverse direction. A net current therefore passes out of the series connected diode stack 120 in the forward direction.
When an equally sized falling edge appears on the counter-electrode 130, introducing negative voltage steps within the series connected diode stack 120, the cathode end diode 150 will be reverse biased, and will pass little current. However, the anode end diode 160 will now be forward biased, and will allow a net forward current to pass into the stack 120.
It is clear that, over a complete noise cycle, a net charge will be ‘pumped’ in the forward direction through the series connected diode stack 120. This net current is indistinguishable from that which arises due to illumination, and therefore corrupts the ambient light measurement.
It should be noted that although the anode of the diode stack has been shown connected to ground in FIGS. 3 to 9, a connection to any dc voltage source would be equally valid.
It should also be noted that, due to the proximity of the diode stack to other layout features, capacitance may be present between nodes within the diode stack and ground (or other dc voltage sources). This is shown in FIG. 9. In the case where either the parasitic capacitance 140 to the noise source or the capacitance 170 to the ground (or dc voltage source) is non-uniformly distributed across the diode stack, the size of the voltage perturbations within the diode stack may vary along its length. Such uneven voltage perturbations may increase the transient forward bias voltages across diodes within the stack, raising the charge pumping current.
The ‘charge pumping’ problem can be lessened by fabrication of a conductive shield layer 180 above the diode stack 120. Electrically, this appears as shown in FIG. 10. Although the conductive shield 180 is still capacitively coupled to the counter-electrode 130, it is also grounded via a low resistance path 190, minimizing the voltage perturbations which occur on it. The waveform appearing on the shield 180 may then resemble that shown in FIG. 11. Only small voltage steps will therefore appear within the diode stack 120 on account of the counter-electrode waveform. As the current flowing in a forward biased diode is strongly dependent upon the magnitude of that forward bias voltage, smaller voltage perturbations within the stack 120 yield a lower charge pumping current. Nonetheless, it may not be possible to reduce the shield resistance to zero and hence the voltage perturbations sufficiently, and so the residual charge pumping current may still be unacceptable.
Alternatively, the charge pumping problem can be eliminated by adoption of a display architecture whereby the counter-electrode voltage is constant. To ensure that the LC material experiences a zero-mean electric field, such architectures periodically invert the polarity of the voltages present on the TFT glass 30, relative to that present on the counter-electrode 130. However, higher absolute voltages must be generated on the TFT glass 30 of such displays, increasing complexity of the drive circuits. The bezel area (the non-display region which houses circuits surrounding the pixel matrix) may also be larger in displays of this architecture.